A facsimile machine is an example of an image processing apparatus. The facsimile machine has to have both image reading capability and image printing capability, and further has to be as small as possible.
With this background, there is already a facsimile machine incorporating such a constitution as shown in FIG. 20 as part of the facsimile machine. The illustrated constitution comprises an integrated image-reading/writing head H and two platen rollers P1, P2. The integrated image-reading/writing head H includes a case 1e having an upper opening fitted with a transparent cover 19e. Inside the case 1e, a light source 3e for illuminating a reading line Se provided on the transparent cover 19e, and a lens 5e are provided. The case 1e has a bottom surface mounted with a substrate 4e. The substrate 4e has an upper surface 40 and a back surface 41 respectively mounted with a plurality of light receiving elements 2e and a plurality of heating elements 8e, linearly and longitudinally of the substrate 4e. 
In the image processing apparatus with the constitution described above, a document D is contacted onto the transparent cover 19e and fed by the platen roller P1. During this feeding process, the document D is illuminated by the light source 3e. Then, light reflected by the surface of the document D is focused by the lens 5e, forming an image of the document D on the array of the light receiving elements 2e. Each of the light receiving elements 2e outputs read image data of the document line by line. On the other hand, a recording paper K which is thermosensible is contacted onto each of the heating elements 8e and fed by the platen roller P2. During this feeding process, selected ones of the heating elements 8e are heated to print the image in the recording paper K line by line.
According to the above facsimile machine, the overall height of the machine can be smaller than a facsimile machine in which the reading head and the printing head are provided separately. Further, the image on the document D can be copied on the recording paper K if print image data is prepared based on the read data outputted from the light receiving elements 2e, and this print image data is outputted to the heating elements 8e. 
However, the above facsimile machine has the following problems.
First, the two platen rollers P1, P2 are disposed so as to sandwich the whole of the integrated image-reading/writing head. Thus, the entire assembly of the integrated image-reading/writing head and the two platen rollers P1, P2 has a large dimension thickness-wise of the integrated image-reading/writing head H. Therefore, the thickness of the facsimile machine as a whole is still large.
Second, when manufacturing the integrated image reading/writing head H, first, the light receiving elements 2e must be mounted on the upper surface 40 of the substrate 4e, and then the substrate 4e must be turned over in order to mount the heating elements 8e on the back surface 41 of the substrate 4e. Further, a wiring pattern relevant to the light receiving elements 2e and another wiring pattern relevant to the heating elements 8e must be formed respectively in the upper surface 40 and the back surface 41 of the substrate 4e. Therefore, productivity in the manufacture of the integrated image reading/writing head H is low.